Sealing at high pressures in, for instance, hydraulic cylinders and similar components, is critical regarding function and reliability for most mobile working machines and heavy mechanized and automatized equipment. In spite of intensive efforts to develop more adequate dynamic seals, these wearing sealing members must still be exchanged one or more times during the life of the cylinder. Inadequate sealing function is also one of the most usual and the most serious cause for breakdowns in hydraulic systems. Current sealing techniques and the inadequacies of seals in meeting the desired function not only result in a short lifetime but also result in several other disadvantages which, in varying degrees, depending on application type, adversely impact upon the function of the product and also limit its use in hydraulic applications. In addition, there are other serious disadvantages such as the influence of high and low temperatures and friction and stic-slip limitations.
Because of the importance of proper sealing and also because of the difficulties incurred in achieving proper sealing, it is common in both relatively demanding, and also in relatively simple, applications to attain the best possible result by requiring close diameter tolerances and very smooth surface finishes on the sealing surface, both for cylindrical tubes and piston rods. The prior art seals, usually made of polymeric materials, involve a small part of the total cost of the component, but the demands imposed on cooperating surfaces to be sealed involve substantial cost as compared to the material costs. Thus, the total cost for sealing a component represents a significant cost item while the cost of the sealing ring itself generally is very low.
The problem of short lifetime for the seals arises from three main causes. Seal fatigue can occur due to pressure and pressure changes, resulting in cracks which result in leakage and breakdown. As a result of pressure forces, the seal can be forced into the gap between the two members to be sealed, with cracks, leakage and breakdown occurring as a consequence. Also, in order to reach low dynamic leakage, the sealing body is usually formed with sealing lips or surface projections which will pierce the liquid film. If this part of the seal becomes worn, the desired pressure peak is not obtained, resulting in increased dynamic leakage.
Prior art seals are accordingly designed to take into account fatigue, and to provide adequate wear resistance for the wearing surfaces of the seal. No significant improvements in seals have, however, been developed during the last 20 to 30 years. However, some new optimized configurations of the sealing ring and the introduction of new materials have improved the fatigue and wear characteristics. Nevertheless, the problem of short seal lifetime still exists, mainly due to the wear of the contacting surface of the seal members.
Even with use of the best wear-resistant materials now available, wear is nevertheless, in most cases, unacceptably great. The reason for this is that the principal function of conventional present-day sealing members is dependent upon providing a very high contact pressure at the lip, to provide the required sealing function. But the effectiveness of such a seal presupposes that the sealing lip is not worn or deformed. The required pressure exerted on the material of the seal and at its contact area, mainly the sealing lip, is the factor that determines wear as well as the setting of the material. It has not been possible as a result of the improvements in the prior art to succeed in developing marked improvements in conventional sealing techniques by various new sealing configuration and material qualities, but only to improve sealing to a varying degree. There is accordingly reasons to believe that current conventional sealing techniques already have been quite fully developed in relation to their potential and cannot be developed further in any really appreciable way.
The high pressure exerted at the sealing lip consists of the sum of the liquid pressure and also the pressure that arises in the sealing material when the cross-section of the sealing member is compressed in the sealing groove. Pre-compression of the sealing member is necessary in order to seal dynamically at low pressures and zero pressures. Since the sealing member, with the passage of time, loses its pre-stress, and thereby also of course in the sealing lip, sealing problems often arise at low pressures after a time. These problems usually become acute during wintertime when the viscosity is high and the sealing material at low temperatures has decreased elasticity.
In order to decrease the problems associated with setting, seals made of polymeric materials must accordingly be configured with comparatively large radial height. This results in unwanted large dimensions, and increases the difficulty and expense in incorporating such seals into the product.
In summary, the sealing function of conventional polymeric sealing members at low pressure must be achieved by precompression of a relatively high sealing section. The surface pressure demanded in the sealing lip is usually about 2-5 MPa. The pressure-caused setting and wear in the sealing lip is, for a conventional 25 MPa system, between about 10 and 30 MPa, essentially above the surface pressure required for sealing. It is, accordingly, a fundamental disadvantage in conventional seals that they are loaded far in excess what the seal requires.
A consequence of the high contact pressure between the sealing element and corresponding surface which results from the fluid is that starting friction is increased. Even worse is that this friction is remarkably reduced upon movement with the result that abrupt movement can easily occur. When pressure and viscosity and also a number of other factors are such as to increased the possibility of such an occurence hydraulic actuators often cannot be used where precise positioning is required.
One way to decrease abrasion on a sealing member is to design it so that, at low speed, it forms an oil film and accordingly functions as a slide bearing. For sealing pistons, where internal leakage has no detrimental effect, this method can be used, and it is used also today to a relatively high degree. With a piston rod, however, outer leakage cannot be accepted and measures must therefore be directed toward preventing film formation. If film formation arises, this takes place at increasing speed and viscosity. Since the velocity in a reciprocating movement in a cylinder must decrease to zero for turning, film formation only arises during a part of the stroke. At low velocity, the wear is great and therefore seals having good film formation in practice are not more wear-resistant than seals which cannot form a film.
Thus, the lifetime of conventional seals can be increased by choice of wear-resistant sealing materials and by providing a low profile depth for the cooperating opposite surface, together with a profile that has lower wear than other profile forms. The most usual sealing materials today are polyurethane and nitrile rubber having various wear-decreasing additives. Also PTFE, in combination with other materials, is a relatively common sealing material. Seals made of polymeric materials have, for a long time, most commonly been used as sealing materials in cylinders and similar components. One exception is that piston rings in metal are used relatively often, particularly in the United States. These piston rings are split with a sealingly overlapping slit. The advantages of this sealing type are good life, high temperature resistance, comparatively low friction, good stic-slip qualities, and small dimensions. A great disadvantage, however, is their very high leakage which limits the piston ring to use as a piston sealing, particularly in applications within industrial mechanization.
A piston ring formed of metal should constitute excellent sealing if the leakage is not 100-1000 times higher than that achieved with soft seals in polymeric materials. The fact that piston rings formed of metal nevertheless have been used with great success is explained by the fact that the piston leakage is of approximately the same magnitude as the leakage in the valve guiding the cylinder.
Important differences between the different materials are their stic-slip qualities and friction qualities and also their tendency to form temporary local molecular bonds to the molecules of the opposite surface. In this respect, the usual polyurethane and nitrile materials are the worst, PTFE-combination in different forms is better, and metal generally is best. With the conventional prior art seals of today, there is no seal that in all types of applications is the best, but both seal type and choice of material is selected to achieve the best possible solution.